1. Field of the Invention
This invention relates generally to semiconductor wafer fabrication, and more particularly to semiconductor wafer scrubbing equipment.
2. Description of the Related Art
As is well known, semiconductor devices are fabricated from semiconductor wafers, which are subjected to numerous processing operations. These operations include, for example, impurity implants, gate oxide generation, inter-metal oxide depositions, metallization depositions, photolithography pattering, etching operations, chemical mechanical polishing (CMP), etc. Although these processes are performed in ultra clean environments, the very nature of many of the process operations is to blame for the generation of surface particles and residues. For instance, when CMP operations are performed, a film of particles and/or metal contaminants are commonly left behind.
Because surface particles can detrimentally impact the performance of an integrated circuit device, wafer cleaning operations have become a standard procedural requirement after certain process steps. Although cleaning operations are rather procedural, the equipment and chemicals implemented to perform the actual cleaning are highly specialized. This specialization is important because each wafer, being at different stages of fabrication, represents a significant investment in terms of raw materials, equipment fabrication time, and associated research and development.
To perform the cleaning operations in an automated manner, fabrication labs employ cleaning systems. The cleaning systems typically include one or more brush boxes in which wafers are scrubbed. Each brush box includes a pair of brushes, such that each brush scrubs a respective side of a wafer. To enhance the cleaning ability of such brush boxes, it is common practice to deliver cleaning fluids through the brush (TTB). TTB fluid delivery is accomplished by implementing brush cores that have a plurality of holes that allow fluids being fed into the brush core at a particular pressure to be released into an outer brush surface. The outer brush surface is made out of a very porous and soft material so that direct contact with the delicate surface of a wafer does not cause scratches or other damage. Typically, the outer brush surface is a made out of polyvinyl alcohol (PVA) foam. Although, other materials such as nylon, mohair or a mandrel wrapped with a polishing pad material can be used.
As semiconductor design and performance requirements continue increase, cleaning engineers are also challenged to improve their associated processes. To meet these demands, the same cleaning equipment is now being used to perform operations other than basic de-ionized (DI) water cleaning. Such operations include the application of sophisticated chemicals TTB to remove particulates and/or to etch precision amounts of materials from the surfaces of a wafer. Although much research and development goes into the design of cleaning and etching chemicals, the effectiveness of such chemicals is only as good as their delivery and application onto the surface of a wafer.
Recent research of conventional brush core technology has uncovered non-uniformities in the application of the chemicals onto the surface of wafers. The research indicates that although chemicals are being flushed out of the brush cores and onto the wafer surfaces, the applied chemicals do exit the holes of the brush core at the same rate over the length of a core. For instance, chemicals are generally supplied to an internal bore of a brush core from one end of the brush core at a given pressure. Ideally, the chemicals are expected to flow through the bore and drip or flow out of the core equally from all of the brush core holes (e.g., the same amount drips out each of holes all along the brush core). Unfortunately, research shows that chemicals are not dripping out of all of the holes at the same or substantially the same rate. In fact, much of the research indicates that the brush core holes near the chemical receiving end drip out chemicals at a substantially faster rate than holes at the opposite side of the chemical receiving end.
Because traditional cleaning typically only included the application of DI water and/or ammonia based chemicals, the uneven application of these fluids through the brush core did not in many cases detrimentally impact cleaning performance. However, because most cleaning systems are now required to also apply engineered chemicals, such as hydrofluoric acid (HF) containing etch chemicals, any uneven application will have a severe impact on the wafer being processed. For instance, if more HF is applied to one part of the wafer and less is applied to another part of the wafer, the surface of the processed wafer may exhibit performance impacting etch variations due to experienced chemical concentration variations.
FIG. 1A provides a simplified diagram 10 of a prior art brush core 12 having a plurality of holes 12a. The brush core 12 has a center bore 12b which is configured to receive fluids from a fluid input 16 at one end of the brush core 12. The brush core 12 is shown having a brush 14 mounted thereon to illustrate that fluid that enters the bore 12b exits the holes 12a soaks the brush 14 that is designed to contact a wafer. This simplistic diagram also illustrates fluid flow lines 18a and 18b, in which fluid lines 18a illustrate that more fluid tends to flow out of holes 12a near the fluid input than at the opposite end. It is believed that this occurs because chemicals are either not applied to the brush core 12 at a sufficient pressure or the holes 12 are too large and/or are improperly arranged and thus allow gravity to pull more fluid out of the brush core 12 near the fluid input 16 than at the opposite end.
Some of these prior art brush cores 12 have a center bore 12b that is about 0.36 inch in diameter or larger and holes 12a that are about 0.13 inch in diameter or larger. To compensate for the larger size of these dimensions and to attempt to prevent the uneven delivery of fluids, cleaning systems need to deliver fluids to the brush cores 12 at higher pressures. These higher pressures range between 30 to 35 PSI or higher. However, the application of higher pressures require the cleaning system to have access to facilities and associated equipment that can deliver the desired controlled pressures at all times. However, cleaning systems are installed in clean rooms around the world having different facilities which may or may not be able to deliver the recommended pressures. Additionally, the holes 12a of most prior art brush cores 12 are arranged such that one hole 12a' is directly opposite of another hole 12a'. This arrangement is also believed to contribute to the higher outflow of fluids near the fluid input 16 than at the opposite end.
In view of the foregoing, there is a need for improved brush core designs that enable controlled amounts of fluid to be evenly delivered and distributed over the surface areas of a brush core.